This invention relates to apparatus for cleaning hollow vessels, and more particularly relates to apparatus for removing deposited matter, such as deposited silicon, from the inside of a quartz diffusion tube.
Quartz diffusion tubes are used in the semiconductor industry to carry out chemical vapor deposition and epitaxy deposition. During a chemical vapor deposition or epitaxy process, silicon wafers are enclosed within the quartz diffusion tube and then vapors are introduced into the tube. These vapors act on the silicon wafers to cause the desired features to be formed on them. However, these processes also cause the inner surface of the tube to be covered with a layer of silicon as well as other unwanted by-products. Before the quartz tube can be used again, this contaminating layer must be removed. Conventionally this is done by immersing the tube in a vat containing a reagent, usually a mixture of nitric acid and hydrofluoric acids, and then rinsing the tube thoroughly in a vat of deionized water.
Such a conventional cleaning operation, even though it has been necessary in preparing the tube for subsequent use, is dangerous to human technicians, shortens the life of the tube, and consumes a great amount of floor space as well as large quantities of reagent.
Conventionally, a technician must pick the diffusion tube up by hand and then carefully lay it into a vat containing ten gallons or more of nitric and hydrofluoric acids. These acids are highly corrosive and can cause severe and painful chemical burns if accidentally brought into contact with human skin. If hydrofluoric acid reaches certain sensitive regions, such as the skin under the fingernails, extremely painful sores result. Despite the fact that the technician will take all the usual precautions, such as the wearing of rubber gloves, and that he will exercise the greatest care while handling the diffusion tubes, such chemical burns will inevitably occur.
Further, the reaction of nitric acid with the silicon gives off toxic nitrous oxide gas. Even though the cleaning is carried out under a ventilation hood, there is still a danger to the health of the technician.
Because the quartz tubes are manually lifted into and out of the chemical vats, there is also a danger of breakage to the diffusion tubes. As these tubes are rather expensive, it would be desirable to minimize the handling of the tubes and thereby reduce the probability of breakage.
The acid in the vat etches both the inside and the outside of the diffusion tube, even though only the inside needs to have the silicon removed. In each cleaning, the acid etches away some of the quartz from the diffusion tube, and reduces the remaining life of the diffusion tube. Etching the exterior of the tube unnecessarily shortens the tube life.
Furthermore, the vat of acid, the vat of deionized water, and the hood require an arrangement that occupies a relatively large floor space, usually a space of at least eight feet by three feet.